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CORDIS - Forschungsergebnisse der EU

Electric field imaging of single molecular charges by a quantum sensor

Periodic Reporting for period 4 - SMel (Electric field imaging of single molecular charges by a quantum sensor)

Berichtszeitraum: 2022-02-01 bis 2022-07-31

The sensitive detection of physical parameters is a key technical challenge in a wide variety of application, from autonomous cars to medical diagnostics. Novel sensors, specifically when based on quantum effects, do have the potential to achieve record breaking sensitivity and selectivity. The aim of SMeL is improve the detection of electric fields and apply it to material science. Specifically, SMeL is geared towards exploiting the outstanding nanoscale quantum sensing capabilities of spin defects in diamond. By specifically tailoring the spin states of the diamond defects -most notably that of the nitrogen vacancy (NV) center in diamond- by coherent quantum control, the project aims to maximize the sensitivity of the spin quantum states to electric fields and achieve highly sensitive charge detection down to single elementary charges under ambient conditions. While detecting magnetic fields with the quantum sensor is a widely applied technology, sensing of electric field is scarcely applied. SMeL intends to develop a nanoscale imaging technique that achieves this goal to study molecular properties and structures based on the measured electric fields. This is achieved by a dedicated Hamiltonian engineering of the sensor spin. In preliminary work, the group of the applicants has adapted a technique known from high resolution magnetometry with NV spins to provide the basis for high sensitivity detection of electric fields. The overall achievement of the project is the achievement of record-breaking sensitivity in electric field detection. Based on the technique used for this achievement, the grantee was also taking a next step in using nanoscale NMR to identify the chemical composition of molecules in nanoscale volumes. The grantee was for the first time able to demonstrate nanoscale NMR on minute amounts of molecules. Among others, this achievement has been enabled by mitigating the influence of diffusion on the spectral resolution, i.e. the linewidth of the NMR spectra. The results achieved in SMeL do have the potential to revolutionize chemical and biochemical structure analysis by achieving a more that 10 order of magnitude improvement over comparable classical methods.
Quantum sensors based on diamond spin systems are on their way to become major tools in material science. While the detection of magnetic fields generated from ferro- or antiferromagnetic structures is routinely done, this is not the case for electric fields. A major part of the work accomplished in SMeL was thus to invent methods to enhance the sensitivity of the quantum sensor for electric fields. It should be noted, that the spin levels of the NV center are not sensitive to electric fields in first order. We have carried out systematic investigations of various experimental conditions to maximize electric field sensitivity. As a result, we were able to achieve record-breaking electric field sensitivities. We were able to apply this technique to nanoscale detection and imaging of electric fields. This is of particular relevance as in most materials, either used in electronics or in biostructures strong local electric fields exist. Those fields are shielded macroscopically but exist on small length scales. In SMeL we were able to detect those fields in model materials and even provide image of local electric fields with nanometer resolution.
To improve electric field sensitivity, the grantee was also investigating quantum systems in other host materials like silicon carbide that, like diamond defects, carries a spin and do show optically detected spin resonance. Those spins can be used for quantum sensing, albeit with less sensitivity due to the lower signal contrast. This is in part, especially for electric field sensing, compensated by a larger susceptibility to electric fields. The grantee has investigated its quantum properties and derived optimum parameters for quantum sensing experiments. These results pave the way for an integrated quantum device based on SiC which is currently explored with leading companies in Europe.

Temperature sensing is an attractive feature of the NV quantum sensor. Its physics is very similar to electric field sensing. However, by dedicated quantum control methods, the impact of temperature changes can be separated from electric field induced level shifts. The beauty of temperature sensing with spin quantum sensors is, that small temperature changes can be measured at very small length scales, e.g. a few nm. This allows to measure temperature gradients in e.g. living cells and this provide insight into biochemical processes in these structures. A major task was to achieve sufficient sensitivity to measure small temperature differences. The applicant was able to demonstrate the so far best temperature sensitivity of a few 10 mK/√Hz.
By improving sample material and developing novel imaging techniques, as well as fabricating we achieved high resolution imaging of nuclear spins using a technique called wide-field nuclear magnetic resonance imaging. In this approach, we combine imaging of diamond defects by a wide field fluorescence microscope with the sensing capabilities of the nitrogen vacancy (NV) center. We were demonstrating a sub 200nm spatial resolution in the detection of a nuclear magnetic resonance (NMR) signal of a thin layer of CaF2. This is the so far highest achieve spatial resolution achieved with this method and is a further step towards a versatile application of the technique.
Solid state quantum sensors are a novel tool in micro- and nanoscale materials investigation as well as biomedical science. Extending their range of application is a key challenge in the field. In SMeL we were able to extend the sensing capabilities of diamond quantum sensors extensively and set new record values for the detection of electric fields together with charge state manipulation. Specifically, we showed, that the current limitation in the magnetic field sensitivity of nanoscale diamond quantum sensors results from charge fluctuations on the surface of the sensor. By applying large local electric field, we were able to remove those charges, image the resulting changes in the electric fiend and improve the coherence properties of the sensor by more than one order of magnitude. This provides fundamentally new insight into the sensors material science and improves their sensitivity significantly. We also achieved record sensitivity values in temperature measurements. This is of particular importance for measurement of nanoscale heat transport, as it is of interest in nanoelectronic structures and device characterization as well as cellular structures. A final major advance of the state-of-the-art is the unprecedented sensitivity and spectral resolution of nuclear magnetic resonance experiments we achieved during SMeL. Nanoscale nuclear magnetic resonance is one of the most promising applications of diamond-based quantum sensors. Yet, up until recently its spectral resolution was so low, that no fine structure, characteristic for the application of nuclear magnetic resonance could be resolved. Within SMeL we were able to accomplish nanoscale nuclear magnetic resonance with chemical shift resolution, a first step towards application of the technique in chemical analytics.